Skip to main content

Thank you for visiting nature.com. You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript.

Attractive and repulsive Fermi polarons in two dimensions

Abstract

The dynamics of a single impurity in an environment is a fundamental problem in many-body physics. In the solid state, a well known case is an impurity coupled to a bosonic bath (such as lattice vibrations); the impurity and its accompanying lattice distortion form a new entity, a polaron. This quasiparticle plays an important role in the spectral function of high-transition-temperature superconductors, as well as in colossal magnetoresistance in manganites1. For impurities in a fermionic bath, studies have considered heavy or immobile impurities which exhibit Anderson’s orthogonality catastrophe2 and the Kondo effect3. More recently, mobile impurities have moved into the focus of research, and they have been found to form new quasiparticles known as Fermi polarons4,5,6,7. The Fermi polaron problem constitutes the extreme, but conceptually simple, limit of two important quantum many-body problems: the crossover between a molecular Bose–Einstein condensate and a superfluid with BCS (Bardeen–Cooper–Schrieffer) pairing with spin-imbalance8 for attractive interactions, and Stoner’s itinerant ferromagnetism9 for repulsive interactions. It has been proposed that such quantum phases (and other elusive exotic states) might become realizable in Fermi gases confined to two dimensions10,11. Their stability and observability are intimately related to the theoretically debated12,13,14,15,16 properties of the Fermi polaron in a two-dimensional Fermi gas. Here we create and investigate Fermi polarons in a two-dimensional, spin-imbalanced Fermi gas, measuring their spectral function using momentum-resolved photoemission spectroscopy17,18,19. For attractive interactions, we find evidence for a disputed pairing transition between polarons and tightly bound dimers, which provides insight into the elementary pairing mechanism of imbalanced, strongly coupled two-dimensional Fermi gases. Additionally, for repulsive interactions, we study novel quasiparticles—repulsive polarons—the lifetime of which determines the possibility of stabilizing repulsively interacting Fermi systems.

Your institute does not have access to this article

Relevant articles

Open Access articles citing this article.

Access options

Buy article

Get time limited or full article access on ReadCube.

$32.00

All prices are NET prices.

Figure 1: Polaron energy level diagram and measured single particle spectral function.
Figure 2: Attractive polaron.
Figure 3: Finite temperature and impurity concentration.
Figure 4: Repulsive polaron.

References

  1. Devreese, J. T. & Alexandrov, A. S. Fröhlich polaron and bipolaron: recent developments. Rep. Prog. Phys. 72, 066501 (2009)

    ADS  Article  Google Scholar 

  2. Anderson, P. W. Infrared catastrophe in Fermi gases with local scattering potentials. Phys. Rev. Lett. 18, 1049–1051 (1967)

    CAS  ADS  Article  Google Scholar 

  3. Kondo, J. Resistance minimum in dilute magnetic alloys. Prog. Theor. Phys. 32, 37–49 (1964)

    CAS  ADS  Article  Google Scholar 

  4. Prokof'ev, N. & Svistunov, B. Fermi-polaron problem: diagrammatic Monte Carlo method for divergent sign-alternating series. Phys. Rev. B 77, 020408 (2008)

    ADS  Article  Google Scholar 

  5. Schirotzek, A., Wu, C.-H., Sommer, A. & Zwierlein, M. W. Observation of Fermi polarons in a tunable Fermi liquid of ultracold atoms. Phys. Rev. Lett. 102, 230402 (2009)

    ADS  Article  Google Scholar 

  6. Nascimbène, S. et al. Collective oscillations of an imbalanced Fermi gas: axial compression modes and polaron effective mass. Phys. Rev. Lett. 103, 170402 (2009)

    ADS  Article  Google Scholar 

  7. Kohstall, C. et al. Metastability and coherence of repulsive polarons in a strongly interacting Fermi mixture. Preprint at http://arXiv.org/abs/1112.0020 (2011)

  8. Chevy, F. & Mora, C. Ultra-cold polarized Fermi gases. Rep. Prog. Phys. 73, 112401 (2010)

    ADS  Article  Google Scholar 

  9. Duine, R. A. & MacDonald, A. H. Itinerant ferromagnetism in an ultracold atom Fermi gas. Phys. Rev. Lett. 95, 230403 (2005)

    CAS  ADS  Article  Google Scholar 

  10. Chubukov, A. V. Kohn-Luttinger effect and the instability of a two-dimensional repulsive Fermi liquid at T = 0. Phys. Rev. B 48, 1097–1104 (1993)

    CAS  ADS  Article  Google Scholar 

  11. Conduit, G. J., Conlon, P. H. & Simons, B. D. Superfluidity at the BEC-BCS crossover in two-dimensional Fermi gases with population and mass imbalance. Phys. Rev. A 77, 053617 (2008)

    ADS  Article  Google Scholar 

  12. Parish, M. M. Polaron-molecule transitions in a two-dimensional Fermi gas. Phys. Rev. A 83, 051603 (2011)

    ADS  Article  Google Scholar 

  13. Zöllner, S., Bruun, G. M. & Pethick, C. J. Polarons and molecules in a two-dimensional Fermi gas. Phys. Rev. A 83, 021603 (2011)

    ADS  Article  Google Scholar 

  14. Klawunn, M. & Recati, A. Fermi polaron in two dimensions: Importance of the two-body bound state. Phys. Rev. A 84, 033607 (2011)

    ADS  Article  Google Scholar 

  15. Schmidt, R., Enss, T., Pietilä, V. & Demler, E. Fermi polarons in two dimensions. Phys. Rev. A 85, 021602(R) (2012)

    ADS  Article  Google Scholar 

  16. Ngampruetikorn, V., Levinsen, J. & Parish, M. M. Repulsive polarons in two-dimensional Fermi gases. Preprint at http://arXiv.org/abs/1110.6415 (2011)

  17. Dao, T.-L., Georges, A., Dalibard, J., Salomon, C. & Carusotto, I. Measuring the one-particle excitations of ultracold Fermionic atoms by stimulated Raman spectroscopy. Phys. Rev. Lett. 98, 240402 (2007)

    ADS  Article  Google Scholar 

  18. Stewart, J. T., Gaebler, J. P. & Jin, D. S. Using photoemission spectroscopy to probe a strongly interacting Fermi gas. Nature 454, 744–747 (2008)

    CAS  ADS  Article  Google Scholar 

  19. Feld, M., Fröhlich, B., Vogt, E., Koschorreck, M. & Köhl, M. Observation of a pairing pseudogap in a two-dimensional Fermi gas. Nature 480, 75–78 (2011)

    CAS  ADS  Article  Google Scholar 

  20. Cui, X. & Zhai, H. Stability of a fully magnetized ferromagnetic state in repulsively interacting ultracold Fermi gases. Phys. Rev. A 81, 041602 (2010)

    ADS  Article  Google Scholar 

  21. Pietilä, V., Pekker, D., Nishida, Y. & Demler, E. Pairing instabilities in quasi-two-dimensional Fermi gases. Phys. Rev. A 85, 023621 (2012)

    ADS  Article  Google Scholar 

  22. Lobo, C., Recati, A., Giorgini, S. & Stringari, S. Normal state of a polarized Fermi gas at unitarity. Phys. Rev. Lett. 97, 200403 (2006)

    CAS  ADS  Article  Google Scholar 

  23. Punk, M., Dumitrescu, P. T. & Zwerger, W. Polaron-to-molecule transition in a strongly imbalanced Fermi gas. Phys. Rev. A 80, 053605 (2009)

    ADS  Article  Google Scholar 

  24. Pilati, S., Bertaina, G., Giorgini, S. & Troyer, M. Itinerant ferromagnetism of a repulsive atomic Fermi gas: a quantum Monte Carlo study. Phys. Rev. Lett. 105, 030405 (2010)

    CAS  ADS  Article  Google Scholar 

  25. McGuire, J. B. Interacting Fermions in one dimension. II. Attractive potential. J. Math. Phys. 7, 123–132 (1966)

    CAS  ADS  MathSciNet  Article  Google Scholar 

  26. Fröhlich, B. et al. Radiofrequency spectroscopy of a strongly interacting two-dimensional Fermi gas. Phys. Rev. Lett. 106, 105301 (2011)

    ADS  Article  Google Scholar 

  27. Schmidt, R. & Enss, T. Excitation spectra and rf response near the polaron-to-molecule transition from the functional renormalization group. Phys. Rev. A 83, 063620 (2011)

    ADS  Article  Google Scholar 

  28. Sommer, A. T., Cheuk, L. W., Ku, M. J. H., Bakr, W. S. & Zwierlein, M. W. Evolution of fermion pairing from three to two dimensions. Phys. Rev. Lett. 108, 045302 (2012)

    ADS  Article  Google Scholar 

  29. Levinsen, J. & Baur, S. K. Ground state of an impurity in a quasi-two-dimensional Fermi gas. Preprint at http://arXiv.org/abs/1202.6564 (2012)

  30. Sanner, C. et al. Correlations and pair formation in a repulsively interacting Fermi gas. Preprint at http://arXiv.org/abs/1108.2017 (2011)

  31. Petrov, D. & Shlyapnikov, G. Interatomic collisions in a tightly confined Bose gas. Phys. Rev. A 64, 012706 (2001)

    ADS  Article  Google Scholar 

  32. Bloch, I., Dalibard, J. & Zwerger, W. Many-body physics with ultracold gases. Rev. Mod. Phys. 80, 885–964 (2008)

    CAS  ADS  Article  Google Scholar 

Download references

Acknowledgements

We thank S. Baur, N. Cooper, E. Demler, T. Enss, C. Kollath, J. Levinsen, M. Parish, R. Schmidt and W. Zwerger for discussions, and J. Bohn for communicating unpublished details of the |−5/2〉/|−3/2〉 Feshbach resonance in 40K. The work was supported by EPSRC (EP/G029547/1), Daimler-Benz Foundation (B.F.), Studienstiftung and DAAD (M.F.).

Author information

Authors and Affiliations

Authors

Contributions

The experimental set-up was devised and constructed by M.F., B.F., E.V. and M. Köhl, data taking was performed by M. Koschorreck, E.V. and D.P., data analysis was performed by M. Koschorreck and D.P., and the manuscript was written by M. Köhl with contributions from all co-authors.

Corresponding author

Correspondence to Michael Köhl.

Ethics declarations

Competing interests

The authors declare no competing financial interests.

Supplementary information

Supplementary Information

This file contains Supplementary Figures 1-4. (PDF 339 kb)

PowerPoint slides

Rights and permissions

Reprints and Permissions

About this article

Cite this article

Koschorreck, M., Pertot, D., Vogt, E. et al. Attractive and repulsive Fermi polarons in two dimensions. Nature 485, 619–622 (2012). https://doi.org/10.1038/nature11151

Download citation

  • Received:

  • Accepted:

  • Published:

  • Issue Date:

  • DOI: https://doi.org/10.1038/nature11151

Further reading

Comments

By submitting a comment you agree to abide by our Terms and Community Guidelines. If you find something abusive or that does not comply with our terms or guidelines please flag it as inappropriate.

Search

Quick links

Nature Briefing

Sign up for the Nature Briefing newsletter — what matters in science, free to your inbox daily.

Get the most important science stories of the day, free in your inbox. Sign up for Nature Briefing